The present invention relates to processing methods and apparatus for converting coal or other carbon-bearing materials into char. Char can be produced by heating coal or other carbon-bearing materials to selected temperatures in a reduced-oxygen environment. Char having suitable properties may be used in, among other things, iron and steel processing furnaces.
Heating coal or other carbon-bearing materials in a reduced-oxygen environment produces coal gas, volatile liquids and a residue of char. During the process of making char, volatile materials, such as hydrocarbon fuels, in the carbon-bearing materials fluidize when heated to a temperature of approximately 650° F. (approximately 350° C.) and higher.
A pyrolyzer furnace is one apparatus that may be used for processing coal and other hydrocarbon materials into char. A pyrolyzer can operate in a batch or in a continuous process. In one continuous pyrolyzer, one or more drive screws rotate within the pyrolyzer furnace, wherein the coal is heated in a reduced-oxygen environment to a temperature to fluidize the volatile material as the carbon-bearing materials are moved through the furnace. An example of a continuous pyrolyzer furnace is disclosed in U.S. Pat. No. 5,151,159 to Wolfe, et al. Previous pyrolyzer furnaces disclosed by the prior art had heating elements positioned within the furnace housing, which generated hot spots within the furnace, caused uneven heating of the coal or other carbon-bearing material, and caused fatigue and shortened the life of the furnace components.
Another limitation has been the energy efficiency of previous pyrolyzer furnaces. The previous pyrolyzer furnaces were typically heated by electric heaters, or by burning natural gas, fuel oil or propane, to process the fluidized volatile material into hydrocarbon fuel and coal tar products. Pyrolyzer furnaces in the prior art also had drive screws with solid shafts, oil cooled shafts, and other shaft configurations that were thermally inefficient, resulting in the pyrolyzer furnace consuming more fuel.
What has been needed is a pyrolyzer furnace system, and method for making char in that system, that substantially reduces the external energy, e.g. propane, fuel oil, or natural gas, needed for the char making process. The level of additional energy may be reduced to a point that the char making process is sustained by burning only the fluidized volatile materials generated from char making after start up.
Disclosed is a char making apparatus comprising:                a. a longitudinal pyrolyzer furnace housing wherein carbon-bearing material containing volatile materials may be heated to a temperature to fluidize volatile materials therein;        b. at least two counter rotatable drive screws laterally positioned and overlapping within the longitudinal furnace housing, and capable of conveying carbon-bearing materials containing volatile material through the pyrolyzer furnace housing, each drive screw having a hollow drive shaft and a diverter longitudinally positioned within the drive shaft, the diverter forming with an inner surface of each drive shaft an inner passageway capable of directing heated fluid adjacent the carbon-bearing materials moving through the pyrolyzer furnace to fluidize the volatile material therein;        c. a combustion chamber capable of burning fluidized volatile material and, if desired, other hydrocarbon fuels, and exhausting combustion fluids through the inner passageway within the hollow drive shaft of the rotatable drive screws within the pyrolyzer furnace housing; and        d. a conduit being capable of transferring fluidized volatile material from the pyrolyzer furnace to the combustion chamber to be burned.        
Also disclosed is a method for making char, comprising the steps of:                a. assembling a longitudinal pyrolyzer furnace housing having at least two counter rotatable drive screws laterally positioned and overlapping within the longitudinal furnace housing, and capable of conveying carbon-bearing materials containing volatile material through the pyrolyzer furnace housing, each drive screw having a hollow drive shaft and a diverter longitudinally positioned within the drive shaft, the diverter forming with an inner surface of each drive shaft an inner passageway capable of directing heated fluid adjacent the carbon-bearing materials moving through the pyrolyzer furnace to fluidize the volatile material therein;        b. assembling a combustion chamber adjacent the longitudinal pyrolyzer furnace housing capable of burning fluidized volatile material and, if desired, other hydrocarbon fuels, and exhausting combustion fluids through the inner passageway within the hollow drive shaft of the drive screws within the pyrolyzer furnace housing; and        c. counter rotating the screws to cause carbon-bearing material containing volatile materials to move through the longitudinal pyrolyzer furnace housing and be heated to a temperature to fluidize volatile materials therein.        
The fluidized volatile material may be transferred from the pyrolyzer furnace to the combustion chamber, where the fluidized volatile material may be burned to provide some or all of the heat needed to fluidize volatile material in the pyrolyzer furnace. The char making furnace, and method of operation thereof, may be capable of heating volatile material in the carbon-bearing material to a temperature within the range of approximately 650° F. to 1300° F. The combustion fluids exhausted through the inner passageways may also flow in the same direction as the drive screws move the carbon-bearing material through the pyrolyzer furnace housing.
The pyrolyzer furnace may comprise a double outer wall at least partially around the drive screws and forming an outer passageway between the outer walls capable of conveying a flow of heated fluid adjacent the carbon-bearing material moving through the pyrolyzer furnace to fluidize the volatile material therein. A device, such as protrusions, tabs, ribs or other shapes, may provide a turbulent flow of combustion fluids through the inner passageway, and if present, the outer passageway, at a Reynolds number greater than 4000. Further, at least one manifold conduit may conduct heated fluid from the combustion chamber to selected portions of the outer passageway along the pyrolyzer furnace housing.
Alternately or in addition, at least one clearing screw having a smaller diameter may be positioned longitudinally through the furnace housing adjacent the drive screws, and capable of conveying carbon-bearing materials from the drive screws through the pyrolyzer furnace housing.
Also, the pyrolyzer furnace may have at least three drive screws laterally positioned within the pyrolyzer furnace housing, the drive screws being positioned such that each screw overlaps at least one other screw. If desired, more than one clearing screw may be positioned adjacent the drive screws and capable of conveying carbon-bearing materials from the drive screws through the pyrolyzer furnace housing.
A portion of the pyrolyzer furnace housing through which the carbon-bearing material moves may comprise a decreasing cross sectional area in the portion through which the carbon-bearing material moves in the direction of travel of the carbon-bearing material. To accomplish this, at least a portion of the pyrolyzer furnace housing may have a tapered outer wall in the direction of travel of the carbon-bearing material through the pyrolyzer furnace housing, and/or the outer wall of the hollow drive shaft of the drive screws may have a taper to reduce the cross sectional area in the direction of travel of the carbon-bearing material.
In addition, the pyrolyzer furnace may have a furnace housing comprising a first zone and a second zone. The first zone is capable of fluidizing volatile material in the carbon-bearing material. The second zone is capable of mixing supplemental materials, e.g. iron oxide-bearing material, with the carbon-bearing material, the supplemental material being introduced into the furnace housing in the second zone.
At least a portion of the pyrolyzer furnace housing may rotate around the drive screws.